Program to find the index of first Recurring Character in the given string in Python

In string processing, we often need to find the first recurring character ? the first character that appears again in the string. This problem requires us to track which characters we've seen and return the index where we encounter a character for the second time.

So, if the input is like "abcade", then the output will be 3, as 'a' appears first at index 0 and recurs at index 3.

Approach

To solve this, we will follow these steps ?

• Create a set to store characters we've already seen
• Iterate through each character with its index
• If the character is already in our set, return the current index
• Otherwise, add the character to the set
• If no recurring character is found, return -1

Using Set (Efficient Approach)

This approach uses a set to track seen characters, providing O(n) time complexity ?

def find_first_recurring_index(s):
    seen = set()
    
    for i in range(len(s)):
        if s[i] in seen:
            return i
        seen.add(s[i])
    
    return -1

# Test with example
result = find_first_recurring_index("abcade")
print(f"Index of first recurring character: {result}")

# Test with no recurring characters
result2 = find_first_recurring_index("abcdef")
print(f"No recurring character: {result2}")

Index of first recurring character: 3
No recurring character: -1

Using Dictionary (Alternative Approach)

This approach uses a dictionary to count occurrences, though less efficient for this specific problem ?

from collections import defaultdict

def find_first_recurring_with_dict(s):
    chars = defaultdict(int)
    
    for i in range(len(s)):
        if s[i] in chars:
            return i
        else:
            chars[s[i]] += 1
    
    return -1

# Test the function
result = find_first_recurring_with_dict("abcade")
print(f"Result: {result}")

# Test with string "programming"
result2 = find_first_recurring_with_dict("programming")
print(f"Result for 'programming': {result2}")

Result: 3
Result for 'programming': 3

Comparison of Approaches

Note: k is the number of unique characters in the string

Edge Cases

def find_first_recurring_index(s):
    seen = set()
    
    for i in range(len(s)):
        if s[i] in seen:
            return i
        seen.add(s[i])
    
    return -1

# Test edge cases
test_cases = [
    "",           # Empty string
    "a",          # Single character
    "aa",         # Immediate repetition
    "abcdef",     # No repetition
    "abcabc"      # Multiple repetitions
]

for test in test_cases:
    result = find_first_recurring_index(test)
    print(f"'{test}' ? {result}")

'' ? -1
'a' ? -1
'aa' ? 1
'abcdef' ? -1
'abcabc' ? 3

Conclusion

The set-based approach is most efficient for finding the first recurring character, offering O(n) time complexity. Use a dictionary approach when you need additional character frequency information beyond just finding the first recurrence.